1. Field of the Invention
The present invention relates generally to a lithium secondary battery provided with a positive electrode capable of intercalating and eliminating lithium ions, a negative electrode capable of intercalating and eliminating lithium ions, and an electrolyte, and more particularly, to a lithium secondary battery whose charge/discharge cycle performance and storage characteristics in a charged state are improved upon improvement of its electrolyte.
2. Description of the Related Art
In recent years, secondary batteries have begun to be used in various fields such as electronic equipment and the like. A lithium secondary battery utilizing oxidation and reduction of lithium ions is attracting great attention as one of new-type secondary batteries having high power and high energy density.
Such a lithium secondary battery generally employs a carbon material capable of intercalating and eliminating lithium ions; a lithium metal; and a lithium alloy as a negative electrode active material for its negative electrode, a lithium-containing transition metal oxide such as LiCoO2, LiNiO2, LiMn2O4, or LiFeO2 as a positive electrode active material for its positive electrode, and a non-aqueous electrolyte solution obtained by dissolving a solute of various lithium salts in an organic solvent; a polymer electrolyte comprising a polymer impregnated with a solute; and a gelated polymer comprising a polymer impregnated with a solute and an organic solvent as an electrolyte.
Examples of an organic solvent to be used in the above-mentioned electrolyte generally include ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane, and the like. These solvents may be used alone or in combination of two or more types. Further, examples of a solute to be used in the above-mentioned electrolyte generally include LiPF6, LiBF4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, LiC(CF3SO2)3, LiCF3(CF2)3SO3, and the like.
However, in such a lithium secondary battery, charging and discharging are performed at high voltage. Accordingly, reaction between the above-mentioned negative electrode and electrolyte is induced, whereby particularly a solvent in the electrolyte is degraded upon decomposition. Therefore, a problem exists that the lithium secondary battery suffers degraded charge/discharge cycle performance and storage characteristics.
Therefore, in the prior art, Japanese Patent Laid-Open No. Hei7(1995)-85888 has proposed an electrolyte obtained by dissolving as a solute an imide group lithium salt represented by Li(CnX2n+1Y)2N (wherein X denotes halogen, n denotes an integer of 1 to 4, and Y denotes a CO group or an SO2 group) in a concentration of 0.1 to 3 mole/liter in a mixed solvent containing at least one type of solvent selected from ethylene carbonate and propylene carbonate at 10 to 80 vol % and at least one type of solvent selected from diethoxyethane, chain carbonate, and acetonitrile at 20 to 90 vol %.
The above-mentioned gazette shows that charge/discharge cycle performance of the lithium secondary battery is improved by the use of such an electrolyte.
However, even in a case where the electrolyte employs as a solute the above-mentioned imide group lithium salt, the electrolyte is in direct contact with a positive electrode and negative electrode and hence, the electrolyte is decomposed upon reaction with a positive electrode active material or negative electrode active material when the battery is in a charged state. Accordingly, there still remains a problem that the battery suffers degraded storage characteristics in a charged state.
An object of the present invention is to attain excellent storage characteristics in a charged state in a lithium secondary battery provided with a positive electrode capable of intercalating and eliminating lithium ions, a negative electrode capable of intercalating and eliminating lithium ions, and an electrolyte by improving the electrolyte so that the electrolyte is prevented from being decomposed by a positive electrode active material or a negative electrode active material when the battery is stored in a charged state.
A lithium secondary battery according to the present invention is a lithium secondary battery provided with a positive electrode capable of intercalating and eliminating lithium ions, a negative electrode capable of intercalating and eliminating lithium ions, and an electrolyte, wherein at least one of an imide group lithium salt represented by LiN(CmF2m+1SO2)(CnF2+1SO2) (wherein m and n each denote an integer of 1 to 4 and may be the same or different from each other) and a methide group lithium salt represented by LiC(CpF2p+1SO2)(CqF2q+1SO2) (CrF2r+1SO2) (wherein p, q, and r each denote an integer of 1 to 4 and may be the same or different from each other) is contained as a chief component of a solute in said electrolyte, and one of or both of a fluoride and phosphorus compound are added to said electrolyte.
As in the lithium secondary battery according to the present invention, when one of or both of a fluoride and phosphorus compound are added to the electrolyte containing at least one of the above-mentioned imide group lithium salt and methide group lithium salt as a chief component of the solute, a protective film is formed on a surface of the positive electrode and/or negative electrode by the fluoride and/or phosphorus compound. The protective film thus formed serves to prevent direct contact between the electrolyte and the positive electrode and/or negative electrode.
As a result, in the lithium secondary battery according to the present invention, the electrolyte is prevented from being decomposed even when the battery is stored in a charged state, whereby the storage characteristics of the battery in a charge state is improved.
As the above-mentioned fluoride to be added to the electrolyte, various types of known fluorides may be used. Specifically, it is preferable to use at least one type of fluoride selected from the group consisting of AgF, CoF2, CoF3, CuF, CuF2, FeF2, FeF3, LiF, MnF2, MnF3, SnF2, SnF4, TiF3, TiF4, and ZrF4. Further, as the above-mentioned phosphorus compound to be added to the electrolyte, various types of known phosphorus compound may be used. Specifically, it is preferable to use at least one type of phosphorus compound selected from the group consisting of LiPO3 and Li3PO4.
In adding one of or both of the fluoride and phosphorus compound to the electrolyte as described above, if an amount of the fluoride and/or phosphorus compound added to the electrolyte is too large, a protective film formed on the surface of the positive electrode and/or negative electrode becomes thick, resulting in increased resistance. On the other hand, if the amount is too small, a sufficient protective film can not be formed on the surface of the positive electrode and/or negative electrode, whereby the electrolyte is decomposed upon reaction with the positive electrode and/or negative electrode. Accordingly, in either one of the cases, storage characteristics of the battery in a charged state are degraded. Therefore, in adding one of or both of the fluoride and phosphorus compound to the electrolyte, the amount of the additive is set preferably in the range of 0.001 to 10.0 wt % and more preferably 0.01 to 5.0 wt % based on the total weight of the electrolyte.
Further, when a gelated polymer electrolyte comprising a polymer impregnated with the above-mentioned solute, additive, and organic solvent is used as the above-mentioned electrolyte, the electrolyte is further prevented from being decomposed upon reaction with the positive electrode and/or negative electrode, whereby the storage characteristics of the battery in a charged state is further improved.
In the lithium secondary battery according to the present invention, a known solvent that has been conventionally generally used may be used as a solvent in the above-mentioned electrolyte. Examples of a usable solvent include ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, sulfolane, tetrahydrofuran, 1,3-dioxolane, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane, and the like. These solvents may be used alone or in combination of two or more types.
Further, as a solute in the electrolyte, the above-mentioned imide group lithium salt or methide group lithium salt may be used together with any other solutes.
Further, as a polymer to be used in the above-mentioned gelated polymer electrolyte, a polymer that has been conventionally generally used may be used. Examples of a usable polymer include polyethylene oxide, polypropylene oxide, cross-linked polyethylene glycol diacrylate, cross-linked polypropylene glycol diacrylate, cross-linked polyethylene glycol methyl ether acrylate, cross-linked polypropylene glycol methyl ether acrylate, and the like.
In the lithium secondary battery according to the present invention, as a positive electrode active material for use in its positive electrode, a known material that has been conventionally generally used may be used. Examples of a usable positive electrode active material include metal compounds capable of occluding and discharging lithium ions, which are represented by metal oxides containing at least one of manganese, cobalt, nickel, iron, vanadium, niobium and the like; and lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFeO2, LiCo0.5Ni0.5O2, and LiNi0.7Co0.2Mn0.1O2.
Further, in the lithium secondary battery according to the present invention, as a negative electrode active material for use in its negative electrode, a known negative electrode material that has been conventionally generally used may be used. Examples of a usable negative electrode active material include carbon materials capable of occluding and discharging lithium ions such as natural graphite, artificial graphite, coke, and calcined products of organic substances; lithium alloys such as an Lixe2x80x94Al alloy, an Lixe2x80x94Mg alloy, an Lixe2x80x94In alloy, and an Lixe2x80x94Alxe2x80x94Mn alloy; and lithium metals.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.